One-component, moisture-curable, isocyanate-based coating formulations are well-known and are widely used. Generally, these compositions are based on isocyanate-terminated prepolymers and have a viscosity sufficiently high as to make application to substrates, especially by such techniques as spraying, readily achievable only by dilution with a solvent or solvent mixture to solids levels of 50% or less. However, due at least in part to the introduction of the Clean Air Act of 1970, coatings formulators are under pressure to reduce volatile organic compounds ("VOC") to as low a level as technological advances will allow. Therefore, it would be highly desirable to provide coatings with excellent properties associated with moisture-curable, isocyanate-based coatings, but which are inherently low in viscosity and require minimal or no dilution with solvent.
The use of one-component, isocyanate-based coating compositions in the production of coatings for substrates is known in the art. By way of illustration, U.S. Pat. No. 4,456,658 discloses a process for coating polyvinyl chloride sheet products with a clear coating comprising a binder which consists essentially of at least one polyisocyanate which is liquid at room temperature. The polyisocyanate binder is further characterized at column 2, lines 12-17 of the '658 patent as being in particular polyisocyanates containing biuret or isocyanurate groups and optionally uretidione groups and having an average isocyanate functionality above 2, preferably from about 2.5 to 6. Unfortunately, the '658 patent is limited to coating polyvinyl chloride sheet products, which represents a narrow market niche as compared to the wide variety of substrates in the marketplace that could benefit from a suitable one-component coating. The coating compositions of the examples of the '658 patent also have the disadvantage of slow drying times when cured at room temperature.
As an additional illustration, U.S. Pat. No. 4,292,350 discloses a process of coating substrates comprising applying a solvent-free or low solvent lacquer composition containing as a binder an isocyanate mixture. The isocyanate mixture described in the '350 patent is a mixture of (a) an organic polyisocyanate having an average isocyanate functionality of greater than 2 which contains biuret, urethane and/or isocyanurate groups and has an isocyanate content of from about 13 to 30% by weight, and (b) at least one monoisocyanate having a defined structure. The monoisocyanate is further described at column 3, lines 18-22 of the '350 patent as being suitably prepared by reacting excess quantities of a diisocyanate with a hydrocarbon-containing hydroxyl compound, followed by removal of unreacted excess diisocyanate by distillation using, for example, a thin layer evaporator. The process of this patent is more cumbersome than might be desired, particularly in regard to the need for the isocyanate mixture and the need for removal of excess diisocyanate from the prepared monoisocyanate component. In addition, the inclusion of monoisocyanate provides a chain-stopper and prevents a build-up of high molecular weight during the coating cure.
As yet another illustration, U.S. Pat. No. 5,144,031 discloses discloses both heat-crosslinkable, one-component blocked-isocyanate compositions and two-component coating compositions which are said to harden at room temperature or at a slightly elevated temperature. The coating compositions of the '031 patent contain isocyanurate group-containing polyisocyanates produced by the steps of: (a) partially trimerizing the isocyanate groups of a cycloaliphatic diisocyanate in the presence of a trimerization catalyst and optionally in the presence of an inert solvent, (b) terminating the trimerization reaction at the desired degree of trimerization, (c) removing unreacted starting diisocyanate and any other volatile constituents, and (d) prior to step (c), adding 1 to 30 % by weight of a monohydric alcohol, based upon the weight of the diisocyanate. Unfortunately, this process is more complicated than might be desired, particularly with the requirement for the step (c) removal of unreacted starting diisocyanate after addition of the monohydric alcohol.
As yet another illustration, U.S. Pat. No. 4,801,675 discloses the preparation of prepolymers containing free isocyanate groups and having an isocyanate content of 1 to 16% by weight and said to be useful as binders in one-component lacquers. These prepolymers are made using a polyhydroxyl component having a hydroxyl functionality of about 1.8 to 5.
A simpler process for coating a wide variety of substrates, not limited to the polyvinyl chloride substrate of the '658 patent and not employing the isocyanate mixture of the '350 patent, but which is simpler to carry out than the process of the '031 patent and which utilizes a room temperature coating composition employing a lower functionality polyol than that used in the '675 patent and having a fast drying time would be highly desired by the one-component urethane coatings community. The present invention provides such a simplified process.